DARLING: the store keeps an hourly history of its own pg_stat_statements deltas (V163, part of #5097)

[erikdarlingdata]

What changes for an operator

Once an hour the Darling service records, for the store's own statements, the change in calls, time, rows and blocks since the last snapshot. The 100 statements that spent the most time are kept 90 days in collect.store_statement_history; collect.store_statement_captures says how many were active, how many the extension evicted, and whether the hour was a fresh baseline. Nothing reads it yet; the reader tool is the next PR.

Design in short

• Migration V163: three plain tables (not collectors, not in CollectorCatalog.All); the baseline sits in config so no purge ages it out.
• The hourly store self-metrics tick runs StoreStatementHistory.SnapshotAsync in its own catch (Warning once, then Debug), before the collector-cost flush.
• Deltas are computed in SQL against the stored baseline, so a service restart, store restart or nightly upgrade loses nothing. The epoch is stats_reset: unchanged means subtract; a reset after the previous capture credits the current counters whole; any other change records a new baseline and no history. A counter that fell inside one epoch credits the current value and flags a lower bound.
• Reads only through config.store_statement_stats(); if it is unusable, one precondition capture and no history.
• Viewer schema probe gains the V163 arm; ladder pins updated.

Test plan

• Rung shape, epoch unit cases, SQL pins, command-timeout and worker-source pins.
• Live (scratch store, planted reader functions): first capture, same-epoch delta with idle and new statements across a fresh connection, entry restart, reset inside interval, rebaseline, top-100 cap with statements_seen and hidden count, retention keeping the baseline, precondition capture.
• Live, added after review: an installed but unloaded extension writes a precondition capture; a reader that raises rolls the whole pass back (no history, no capture, baseline unchanged); a same-epoch pass with non-NULL stamps on both sides, where the history row and its capture agree on interval_seconds.
• Source pins, added after review: the snapshot runs under its own linked slice and an expired slice does not skip the flushes; the ready check requires the loaded setting and the TEMP privilege; the top-100 order breaks ties on role name.
• Ladder guards: CaggGroupIndexDrop, plain-table census, alert-read exemptions.
• No MCP tool and no tool description changes, so the tools/list budget does not move. The reader tool comes in a later PR.
• CHANGELOG.md is not edited here; the entry below is spliced at merge.

CHANGELOG

SECTION: Added
ENTRY: - The Darling store keeps an hourly history of its own pg_stat_statements deltas ([#5203]) - migration V163 adds collect.store_statement_history: once an hour the service records, for the store's own statements, the change in calls, time, rows and blocks since the last snapshot. The 100 statements that spent the most time each hour are kept for 90 days, and the history survives service and store restarts and statistics resets.
REF: [#5203]: #5203

Part of #5097